The present invention relates to lasers and more particularly to a laser providing a rapidly sweeping light frequency without laser modes.
Spectroscopic studies evaluate the “spectrographic” response of a material to different frequencies of light. The spectrographic response may be light absorption, reflectivity, scattering, fluorescence or other features.
Spectrographic studies may be used to investigate gases, liquids, aerosols, solids, particulates, and the like, as their physical properties change in response to temperature, pressure, velocity, composition, size, stress and strain. Similar techniques may be used to monitor sensors incorporating materials whose spectrographic responses change as a function of a physical parameter to be measured.
Spectroscopic studies may use a “wavelength-agile” light source providing a spectrally narrow light beam that may be quickly and controllably swept in frequency. One implementation of a wavelength-agile light source employs a laser incorporating a spectral filter into its optical cavity and changing the frequency of that filter during operation of the laser.
One common spectral filter is a diffraction grating. The diffraction grating may be tipped with respect to an incident laser beam to adjust the effective spacing of the grating's rule lines along the beam and hence the frequency of light preferentially reflected by the grating. As the grating is moved to change the frequency of the laser beam, the length of the optical cavity is ordinarily adjusted to match the beam's wavelength to maintain optical resonance. This optical resonance, resulting from standing light waves created by laser cavity elements such as mirrors, is termed a “mode”. At any given laser mode, stimulated emissions by the laser material produce a phase coherence in the emitted light. This phase coherence can produce a phenomenon termed “speckle” in which light from the laser constructively adds or destructively cancels at given points.
A laser system providing simultaneous adjustment of a diffraction grating for frequency selection and optical cavity length to preserve optical resonance is described in U.S. Pat. No. 5,319,668 hereby incorporated by reference.
Spectrographic analysis of short optical phenomena with a wavelength agile laser requires the ability to rapidly change the laser frequency. This speed of frequency change can be limited by mechanical constraints incident to coordinated movement of the optical grating and change in cavity length. At high speeds of frequency change, the laser may “mode hop” jumping from one mode to another mode separated by a substantial wavelength difference. Such a problem is described in U.S. Pat. No. 6,683,895, at col. 2, lines 27 through 30.